The present invention relates to catalyst compositions effective to accelerate curing of aliphatic-isocyanate-based polyurethane compositions. More particularly, it relates to new and improved catalyst compositions consisting essentially of synergistic mixtures of an organic tin compound and an organic lead compound and to an improved method of curing polyurethane compositions employing these catalyst compositions. The present invention also relates to new and improved integral skin foam polyurethane compostions and to improved methods for making useful shaped articles therefrom.
In this application, the term "aromatic isocyanate" refers to an organic isocyanate compound wherein the isocyanate group(s) are bonded directly to a carbon atom of an aromatic nucleus. By "aliphatic isocyanate" is meant an organic isocyanate compound wherein the isocyanate group or groups are bonded directly to an aliphatic carbon atom. The term polyaliphatic isocyanate means a compound having more than one aliphatic-isocyanate linkage in one molecule.
Generally, polyurethane resins and products are obtained by reacting isocyanate compounds and hydroxyl-bearing compounds to form urethane groups in accordance with the equation: EQU RNCO+R'OH.fwdarw.RNHCOOR'
Polyurethane resins are formed by reaction of polyfunctional starting materials containing isocyanate and hydroxyl groups.
Polyurethane resins and articles prepared therefrom are extremely useful because upon curing they can be flexible or rigid and articles prepared therefrom are generally characterized by good solvent resistance, abrasion resistance, impact resistance and colorability.
It is presently known to use polyurethanes to form integral skin foam articles which comprise a core of microcellular foam and a skin which is either microcellular or non-cellular. The prior art polyurethane integral skin foam compositions generally comprise an aromatic isocyanate, such as bis(isocyanatophenyl)methane, a polyol, and a foaming agent. Aromatic isocyanates have been employed because of their high reactivity and rapid cure times. These compositions are castable to form elastomeric articles or may be injection molded in one step in accordance with reaction injection molding (RIM) techniques. The outer surfaces of the skins of these shaped articles can be provided with many and varied decorative features imparted by the inner surfaces of the casting or injection molds, to simulate the appearance of leather or wood materials and the like. Polyurethane integral skin foam products have therefore been extensively used in furniture, automobile, electronics and footwear applications.
A serious disadvantage encountered with these polyurethane compositions and integral skin foam products prepared therefrom is that polyurethanes based on aromatic isocyanates have extremely poor ultraviolet light stability and undesirable yellowing or other discoloration develops with these materials upon exposure to sunlight or other sources of ultraviolet light. In the past, where these compositions were utilized to form colored products, such as, cushions, armrests, seat backs or the like, they had to be colored to an intense shade, or to a dark shade or to black, to hide the undesirable discoloration of the resin. If it was necessary or desired to provide light or brightly colored articles, the articles had to be prepared with a separate non-urethane colored coating layer or had to be dipped or otherwise coated with an ultraviolet light resistant protective layer. These additional coating and production steps are expensive and inefficient in large scale production operations.
More recently, aliphatic isocyanate compounds and polyurethane compositions incorporating them have been developed. The aliphatic isocyanates and their preparation are suitably disclosed in U.S. Pat. No. 2,723,265 and U.S. Pat. No. 3,290,350. Polyurethane compositions based on these aliphatic compounds are known to have excellent ultraviolet light stability and do not discolor upon exposure to sunlight and other sources of ultraviolet radiation. However, the aliphatic isocyanates are considerably less reactive than the aromatic isocyanates. Extended reaction times render aliphatic-isocyanate based polyurethane compositions generally unsuitable for reaction injection molding applications because molding cycle times need to be prohibitively long and the integral skin foam products ultimately obtained are generally soft and exhibit less than desirable strength and load-supporting capability.
Many catalysts have been employed in an effort to accelerate the rate of reaction between aliphatic isocyanates and hydroxyl-containing compounds. Organic lead salts and organic tin salts are two of the more commonly used catalyst materials herefore employed for this purpose. Neither of these catalysts however, when used alone at any reasonable concentration, is effective to adequately accelerate the reaction between secondary and tertiary aliphatic isocyanates and hydroxyl-containing compounds at temperatures below about 100.degree. C., which means that lead catalysts and tin catalysts when used singly are not sufficiently effective to provide a satisfactory RIM moldable aliphatic isocyanate-based polyurethane integral skin foam composition.
In U.S. Pat. No. 4,150,206, a reaction injection moldable polyurethane integral skin foam composition based on an aliphatic isocyanate is disclosed. The compositions are described as possessing a reactivity level substantially similar to the reactivity of aromatic isocyanates. The compositions comprise a polyol, an aliphatic isocyanate, a minor amount of water and a synergistic catalyst combination selected from the following catalyst mixtures:
(i) an amine containing a structure, ##STR2## and a salt, alcoholate and/or phenolate of an alkali metal or alkaline earth metal; PA1 (ii) an organic lead compound and a primary or secondary amine; PA1 (iii) an organic lead compound and an amine containing the structure ##STR3## or (iv) an organic lead compound and a salt, alcoholate or phenolate of an alkali metal or alkaline earth metal. PA1 (a) lead naphthenate and PA1 (b) at least one dialkyltin dicarboxylate compound of the formula: ##STR4## wherein R is C.sub.1 to C.sub.8 alkyl and R' is C.sub.1 to C.sub.18 alkyl, the weight ratio of lead naphthenate to said dialkyltin dicarboxylate in said mixture being from about 25:75 to about 75:25. PA1 (a) a mixture or a prepolymer of: PA1 (b) a minor effective amount of a catalyst composition therefor consisting essentially of a mixture of: PA1 (a) an aliphatic disocyanate compound of the formula: ##STR7## wherein R and R' are each independently, selected from hydrogen, alkyl and substituted alkyl and X is a bridging means selected from divalent aliphatic, cycloaliphatic or aromatic groups, or a prepolymer adduct of said isocyanate compound possessing poly aliphatic isocyanate functionality; PA1 (b) a mixed polyol component comprising: PA1 (c) an effective amount of a crosslinking agent, said crosslinking agent comprising mixture of an amino alcohol and ethylene glycol or a low molecular weight adduct of ethylene oxide and a polyol compound in a weight ratio of from about 1:3 to 3:1, respectively; PA1 (d) a minor effective amount of a catalyst composition therefor, said catalyst composition consisting essentially of: PA1 (e) a foaming agent, wherein the molar ratio of the total isocyanate groups in (A) to the total of hydroxyl groups in (B) and (C) combined is from about 1:0.9 to about 1:1.2, respectively. PA1 (i) about 75 to about 25 parts by weight of lead naphthenate; and PA1 (ii) about 25 to about 75 parts by weight of a dialkyl tin dicarboxylate compound of the formula: ##STR9## wherein R is selected from C.sub.1 to C.sub.8 alkyl and R' is selected from C.sub.1 to C.sub.18 alkyl, per 100 parts by weight of said catalyst composition. PA1 (a) a mixture or a prepolymer of PA1 (b) a minor effective amount of a catalyst composition therefor consisting essentially of the lead naphthenate/dialkyltin dicarboxylate composition described above. PA1 .alpha.,.alpha.'- dimethyl-p-xylylene diisocyanate, PA1 .alpha.,.alpha.,.alpha.',.alpha.'-tetramethyl-m-xylylene diisocyanate, PA1 .alpha.,.alpha.,.alpha.',.alpha.'-tetramethyl-p-xylylene diisocyanate, PA1 bis(4-(1-isocyanato-1-methylethyl) phenyl) methane, PA1 .alpha.-ethyl-.alpha.'-methyl-p-xylylene diisocyanate, PA1 2,6-bis (1-isocyanato-1-methylethyl) naphthalene, PA1 1,4-bis (1-isocyanato-1-methylethyl) cyclohexane, PA1 1,3-bis (1-isocyanato-1-methylethyl) cyclohexane, PA1 1,8-diisocyanato-p-menthane, isophorone diisocyanate, PA1 bis(4-isocyanatocyclohexyl)methane ("H.sub.12 MDI") and the like. PA1 (a) a poly aliphatic isocyanate compound or a prepolymer adduct possessing polyaliphatic isocyanate functionality; PA1 (b) a mixed polyol component comprising: PA1 (c) an effective amount of a crosslinking agent, comprising a mixture of an amino alcohol and ethylene glycol or a low molecular weight adduct of ethylene oxide and a polyol compound in a weight ratio of about 1:3 to about 3:1 respectively; PA1 (d) a minor effective amount of the lead naphthenate/dialkyltin dicarboxylate catalyst composition; and PA1 (e) an effective amount of a foaming agent.
It is disclosed in said patent that if one of the above-identified catalyst mixtures is present that satisfactory reaction rates suitable for RIM processes are obtained and that other catalysts such as tin catalysts, may be added if desired.
Unexpectedly, in view of the foregoing, it has now been discovered that the rate of reaction between aliphatic isocyanate compounds and hydroxyl containing compounds is surprisingly improved by the incorporation of a catalyst composition consisting essentially of certain synergistic mixtures of lead naphthenate and dialkyltin dicarboxylates in specified proportions, said catalyst compositions being added in carefully specified amounts.